1. Field of the Invention
The present invention relates to a semiconductor laser module that is used for optical communication and so on.
2. Description of the Related Art
A development of a high power semiconductor laser module as an exciting light source and the like in an amplifier for an optical fiber that is used for optical communication is in progress.
A conventional semiconductor laser module comprises, as is shown in FIG. 6, for example, a semiconductor laser device 1, an optical fiber 2 that receives laser light emitted from the semiconductor laser device 1 and a photo diode 3 that monitors optical output of the semiconductor laser device 1. These semiconductor laser device 1, optical fiber 2 and photo diode 3 are housed in a package 10.
The semiconductor laser device 1 is optically coupled at its front facet 1a (emitting facet) side with the optical fiber 2 and is confronted at its rear facet 1b (reflecting facet) side with the photo diode 3. An anti-reflection coating film whose reflectance is, for example, around several percent is provided on the front facet 1a of the semiconductor laser device 1 and a reflection coating film whose reflectance is, for example, around 90 percent is provided on the rear facet 1b. 
The semiconductor laser device 1 is fixed to a base (a base part) 6 via a heat sink 9 and a fixing part 5, and the photo diode 3 is fixed to the base 6 by a photo diode fixing part 8.
In the semiconductor laser module shown in FIG. 6, ferules 11a, 11b are provided while keeping a distance from each other in the lengthwise direction of the optical fiber 2, which is inserted and fixed in these ferules 11a, 11b. The ferule 11a functions as a means for supporting the optical fiber and is made of a Kovar (Trade mark) that is a Fexe2x80x94Nixe2x80x94Co alloy, for example.
The base 6 is fixed on a thermomodule 7 and the thermomodule 7 is mounted on a bottom board 10a of the package 10. Here, as is shown in FIG. 6, the thermomodule 7 generally comprises a base-side board 17, a bottom board-side board 18 and a peltier cooler (a peltier device) 19 pinched between these boards 17 and 18. Both the base-side board 17 and the bottom board-side board 18 of the thermomodule 7 are made of Al2O3. And the ferule 11b is fixed to a side-wall of the package 10.
In the above mentioned semiconductor laser module, the semiconductor 1 and the optical fiber 2 are aligned, and laser light emitted from the front facet 1a of the semiconductor laser device 1 is received by the optical fiber 2 to be transmitted in the optical fiber 2 and is provided for a desired use.
Further, in the above mentioned semiconductor laser module, an optical output from the rear facet 1b of the semiconductor laser device 1 is monitored by the photo diode 3, thereby the optical output from the front facet 1a of the semiconductor laser device 1 is controlled. In other words, the output from the emitting facet of the optical fiber 2 is controlled by controlling the laser output based on the monitoring of the photo diode 3.
Here, as the above mentioned semiconductor laser module monitors the optical output from the rear facet 1b of the semiconductor laser device 1 by the photo diode 3, it is needed to permit light having a certain intensity to be emitted from the rear facet 1b of the semiconductor laser device 1 and to reach to a light receiving surface of the photo diode 3. For that purpose, normally, the reflectance of the rear facet 1b of the semiconductor laser device 1 is reduced to a certain extent, for example, to 90 percent or less, intentionally.
However, when the reflectance of the rear facet 1b (reflecting facet) of the semiconductor laser device 1 is reduced, there exists a problem in that an optical output form the front facet 1a of the semiconductor laser device 1 decreases to reduce the optical output from the semiconductor laser module itself.
In recent years, another constitution for a semiconductor laser module has been proposed in which a fiber grating is provided on the optical fiber 2 of the semiconductor laser module. The fiber grating functions as a diffraction grating reflecting only light having a predetermined wavelength out of the laser beams that are emitted from the semiconductor laser device 1 and enter the optical fiber 2. Thus, the output wavelength of the semiconductor laser module is stabilized by providing the diffraction grating, such as a fiber grating, on the optical fiber 2.
However, in this constitution, as the polarization direction of the reflected light selected by the diffraction grating on the basis of the wavelength fluctuates by the change of the positional condition of the optical fiber 2, a returning light with a changed polarization direction is returned to the semiconductor laser device 1 from the front facet 1a of the semiconductor laser device 1 through the optical fiber 2, when the positional condition of the optical fiber 2 is changed. The power of the optical outputs from the front facet 1a and rear facet 1b of the semiconductor laser device 1 thus fluctuates. In particular, the amount of the fluctuation of the optical output by the above mentioned disturbance in the case of light emitted from the rear facet 1b of the semiconductor laser device 1 is bigger than that in the case of light emitted from the front facet 1a. 
Accordingly, when the fiber grating is provided on the optical fiber 2 of the semiconductor laser device 1 shown in FIG. 6, it has been difficult to precisely conduct APC (Automatic Power Control) for controlling optical output from the front facet 1a of the semiconductor laser device 1 to a fixed value. Namely, in the constitution shown in FIG. 6, when the fiber grating is provided on the optical fiber 2 and the output light from the rear facet 1b of the semiconductor laser device 1 is monitored, the fluctuation of monitor current is so large that it was impossible to control the optical output from the front facet 1a of the semiconductor laser device 1 to a fixed value.
The present invention has been made to solve the above mentioned problems, and an object of the present invention therefore to provide a high power semiconductor laser module that can control the optical output to a fixed value.
In order to achieve the above-mentioned object, the present invention provides a semiconductor laser module with the following constitution.
According to a first constitution of the present invention, there is provided a semiconductor laser module comprising: a semiconductor laser device; an optical fiber that receives laser light emitted from the semiconductor laser device; and a photo diode monitoring an optical output of the semiconductor laser device, characterized in that the photo diode monitors an optical output of the semiconductor laser device by receiving scattered light at a laser light receiving end of the optical fiber.
A semiconductor laser module according to a second constitution of the present invention is characterized in that the reflectance at a facet opposite side to the optical fiber of the semiconductor laser device is set to 95 percent or more, in addition to the above mentioned first constitution.
Further, a semiconductor laser module according to a third constitution of the present invention is characterized in that the laser light receiving end of the optical fiber is formed into a lens, in addition to the above mentioned first constitution.
Further, a semiconductor laser module according to a forth constitution of the present invention is characterized in that light emitted from the semiconductor laser device is optically coupled to the laser light receiving end directly, in addition to the above mentioned first (third) constitution.
Further, a semiconductor laser module according to a fifth constitution of the present invention is characterized in that the laser light receiving end is formed into a wedge-shaped lens, in addition to the above mentioned third constitution.
Further, a semiconductor laser module according to a sixth constitution of the present invention is characterized in that the optical fiber includes a diffraction grating reflecting only light having a predetermined wavelength out of laser beams that are emitted from the semiconductor laser device and incident to the optical fiber, in addition to the above mentioned first constitution.
Further, a semiconductor laser module according to a seventh constitution of the present invention further comprises a base on which the semiconductor laser devise is mounted, and is characterized in that the optical fiber is placed such that a plane bisecting the wedge angle of the wedge-shaped lens is parallel to the base and that the photo diode is placed under the wedge-shaped lens, in addition to the above mentioned fifth constitution.
Further, a semiconductor laser module according to an eighth constitution of the present invention further comprises a base on which the semiconductor laser devise is mounted, and is characterized in that the optical fiber is placed such that a plane bisecting the wedge angle of the wedge-shaped lens is perpendicular to the base and that the photo diode is placed on a flank portion side of the laser light receiving end having the wedge-shaped lens, in addition to the above mentioned fifth constitution.
Further, a semiconductor laser module according to a ninth constitution of the present invention further comprises: a base on which the semiconductor laser device and the optical fiber are mounted; a fixing means for fixing the optical fiber to the base; a thermomodule on which the base is mounted; and a package housing the photo diode, the semiconductor laser device, the optical fiber, the fixing means, the base and the thermomodule, and is characterized in that: the thermomodule is mounted on a bottom board of the package; the thermomodule includes a base-side board, a bottom board-side board and a peltier cooler pinched between these boards; the base is composed of a laser device mounting member that is in contact with the thermomodule to mount the semiconductor laser device, and a fixing means mounting member that is placed on a position outside a semiconductor laser device mounting region of the laser device mounting member to mount the fixing means; and the laser device mounting member is made of a material having a coefficient of linear expansion within the range between the coefficient of linear expansion of the fixing means mounting member and the coefficient of linear expansion of the base-side board of the thermomodule, in addition to the above mentioned first constitution.
Further, a semiconductor laser module according to a tenth constitution of the present invention further comprises a base on which the semiconductor laser device and the optical fiber are mounted, and is characterized in that: the optical fiber is fixed to the base while being pinched by a fixing means from both flank portion sides; a package is provided for housing the photo diode, the semiconductor laser device, the base and the fixing means; a thermomodule is mounted on a bottom board of the package; the base is mounted on the thermomodule; a fixing means mounting portion mounting the fixing means is provided on the base; a first laser-welded portion formed by laser-welding the fixing means mounting portion and the fixing means and a second laser-welded portion formed by laser-welding the fixing means and an optical fiber supporting means are provided; and the first laser-welded portion and the second laser-welded portion are made to have approximately the same heights in the direction perpendicular to the bottom board of the package, in addition to the above mentioned first constitution.
Further, a semiconductor laser module according to an eleventh constitution of the present invention further comprises: a base mounting the semiconductor laser device and the optical fiber; a fixing means for fixing the optical fiber to the base; and a thermomodule mounting the base, and characterized in that wall portions are provided on the base at the both side of the frank portions of an optically-coupled portion interposed between a laser light-emitting facet of the semiconductor laser devise and a laser light receiving end of the optical fiber, in addition to the above mentioned first constitution.
Further, a semiconductor laser module according to a twelfth constitution of the present invention further comprises: a base mounting the optical fiber and the semiconductor laser device; a thermomodule mounting the base; and a package housing the photo diode, the semiconductor laser device, the optical fiber, the base and the thermomodule, and is characterized in that: the thermomodule is mounted on a bottom board of the package; the thermomodule includes a base-side board, a bottom board-side board and peltier cooler pinched between these boards; and the difference between the coefficient of linear expansion of base-side board and bottom board-side board and the coefficient of linear expansion of the bottom board of the package is 1xc3x9710xe2x88x926/K or less, in addition to the above mentioned first constitution.
In the present invention according to the above mentioned (first) constitution, a reflection surface is provided at the laser light receiving end of the optical fiber and the photo diode receives laser light (scattered light) reflected at the laser light receiving end of the optical fiber and monitors the optical output of the semiconductor laser module. Therefor the semiconductor laser module according to the present invention does not need to emit light from the rear facet of the semiconductor laser device (the facet on the side opposite to the optical fiber side), unlike the conventional modules.
Accordingly, the semiconductor laser module according to the present invention can increase the optical output of the semiconductor laser device and increase the optical output of the semiconductor laser module itself, as the reflectance at the rear facet of the semiconductor laser device can be increased.
Namely, the semiconductor laser module according to the present invention can, as in the above mentioned second constitution, increase the reflectance at the facet of the semiconductor laser device on the side opposite to the optical fiber to, for example, 95 percent or more, and can be a semiconductor laser module that can achieve a stable and high output.
As the third constitution of the present invention is provided with a lens (for example, wedge-shaped lens or spherical lens) at the laser light receiving end of the optical fiber and the laser light is optically coupled to the lens-like laser light receiving end, the efficiency in collecting light from the semiconductor laser device can be improved and the output of the semiconductor laser module can be increased even more.
Further, when the light emitted from the semiconductor laser device is optically coupled to the laser light receiving end directly, as in the forth constitution of the present invention, the improvement of the light coupling efficiency from the semiconductor laser device of the above mentioned third constitution is achieved effectively.
When the laser light receiving end is formed into a wedge-shaped lens, as in the fifth constitution of the present invention, the processing becomes relatively easy and the module can be applied, with a good collecting efficiency, even to a semiconductor laser device whose emission intensity distribution at its facet is not isotropic (for example, is elliptic) . Further, with this constitution, the laser light can be reflected at slant surfaces of the wedge-shaped lens at the laser light receiving end of the optical fiber, and the inclination of the slant surfaces is large. Therefor, the separation distance between the light emitted from the semiconductor laser device and received at the laser light receiving end of the optical fiber and the scattered light that is the reflection of the received light can be small. Accordingly, in this constitution, the photo diode can be placed in the vicinity of the lensed fiber and the layout design is easy, even when a semiconductor laser device chip carrier and so on are placed together.
Further, in the semiconductor laser module, with a constitution in which, for example, a diffraction grating, such as, a fiber grating reflecting only light having a predetermined wavelength out of all the laser beams is provided on the optical fiber, the oscillation wavelength of the semiconductor laser module can be stabilized.
However, in the constitution in which a diffraction grating is provided on the optical fiber, although the laser output wavelength can be stabilized, there exists a possibility of generating return light, whose reflected light from the diffraction grating fluctuates in its intensity or whose polarization direction fluctuates. Then, in the constitution that a diffraction grating is provided on the optical fiber, the optical output of the semiconductor laser device fluctuates. Even in this case, however, the fluctuation of the optical output emitted from the semiconductor laser device to the optical fiber is smaller than the fluctuation of the light emitted from the rear facet of the semiconductor laser device. Accordingly, the semiconductor laser module of the present invention can easily be controlled by APD control, even when it has the constitution that the diffraction grating is provided on the optical fiber.
Further, by placing the optical fiber such that the plane bisecting the wedge angle of the wedge-shaped lens is parallel to the base mounting the semiconductor laser device and by placing the photo diode under the wedge-shaped lens, as in the seventh constitution of the present invention, the reflected light at the laser light receiving end of the optical fiber can be received and monitored effectively by the photo diode.
Further, by placing the optical fiber such that the plane bisecting the wedge angle of the wedge-shaped lens is perpendicular to the base mounting the semiconductor laser device and by placing the photo diode on the frank portion side of the laser light receiving end of the wedge-shaped lens, as in the eighth constitution of the present invention, the reflected light at the laser light receiving end of the optical fiber can be received and monitored effectively by the photo diode, as in the case of the seventh constitution.
Further, in the present invention, with the constitution that the base mounting the semiconductor laser device and the optical fiber comprises the laser devise mounting member and fixing means mounting member, and the laser device mounting member is made of a material having a coefficient of linear expansion within the range between the coefficient of linear expansion of the fixing means mounting member and the coefficient of linear expansion of the base-side board of the thermomodule mounting the base, the following effect can be obtained.
Namely, with this constitution, even when the thermomodule is bent due to a temperature change of the semiconductor laser module, the warp of the base can be relieved and the deterioration of efficiency of the optical coupling between the semiconductor laser device and the optical fiber due to the temperature change of the working environment can be restrained. Further, as the fluctuation of the light monitored by the photo diode (the reflected light from the optical fiber) is restrained, this constitution can provide a stable and high power semiconductor laser module.
Further, in the present invention, by setting the difference between the coefficient of linear expansion of the base-side board and the bottom board-side board and the coefficient of linear expansion of the bottom board of the package to 1xc3x9710xe2x88x926/K or less, the deterioration of efficiency of the optical coupling between the semiconductor laser device and the optical fiber due to the temperature change of the working environment can be restrained, and a more stable and high power semiconductor laser module can be obtained.
Further, in the present invention, with the constitution that the first laser-welded portion, which is formed by welding the fixing means mounting portion of the base and the fixing means for fixing the optical fiber supporting means with laser, and the second laser-welded portion, which is formed by welding the fixing means and the optical fiber supporting means with laser, have approximately the same height in the direction perpendicular to the bottom board of the package, the effect described below can be obtained.
Namely, with this constitution, even if the base is warped to a certain extent, the position of the optical fiber supporting means does not largely deviate with the first laser-welded portion functioning as a fulcrum, and accordingly, the deterioration of the efficiency of the optical coupling between the semiconductor laser device and the optical fiber can be effectively restrained and the fluctuation of monitoring value by photo diode can be restrained as well.
Further, in the present invention, with the constitution that wall portions are provided on the base on both side of the frank portions of an optically-coupled portion interposed between a laser light-emitting facet of the semiconductor laser devise and a laser light receiving end of the optical fiber, the effect described below can be obtained.
Namely, with this constitution, as the warp of the base at the optically-coupled portion between the semiconductor laser device and the optical fiber can be restrained by the wall portions, the deterioration of the efficiency of the optical coupling between the semiconductor laser device and the optical fiber can be effectively restrained and the fluctuation of monitoring value of light monitored by the photo diode can be restrained as well.